Inherited Change

Question 1

What are transcription factors and what do they do?

Answer 1

1. In both prokaryotes and eukaryotes, transcription of a gene is controlled by transcription factors
2. These are proteins that bind to a specific DNA sequence and control the flow of information from DNA to RNA by controlling the formation of mRNA

Question 2

Whats are structural genes and what are regulatory genes?

Answer 2

• To understand how gene expression in bacteria is controlled, you must distinguish. between structural genes and regulatory genes
  1. Genes that code for proteins required by a cell are called structural genes. Such proteins may literally form part of a cellular structure, but they may also have some other role, such as acting as an enzyme
  2. Genes that code for proteins that regulate the expression of other genes are called regulatory genes

Question 3

What is the difference between repressible and inducible enzymes?

Answer 3

1. The synthesis of a repressible enzyme can be prevented by binding a repressor protein to a specific site, called an operator, on a bacterium’s DNA
2. The synthesis of an inducible enzyme occurs only when its substrate is present. Transcription of the even occurs as a result of the inducer (the enzyme’s substrate) interacting with the protein produced by the regulatory gene

Question 4

What is an operon?

Answer 4

• The different roles of structural and regulatory genes can be seen by looking at the control of gene expression in a prokaryote using the lac operon
• An operon is a length of DNA making up a unit of gene expression in a bacterium
• It consists of one or more structural genes and also control regions of DNA that are recognised by the products of regulatory genes

Question 5

Where does the lac operon act?

Answer 5

1. The enzyme beta-galactosidase hydrolyses the disaccharide lactose to the monosaccharide glucose and galactose
2. In the bacterium, Escherichia coli, the number of molecules of this enzyme present in a bacterial cell varies according to the concentration of lactose in the medium in which the bacterium is growing
3. The bacterium has one copy of the gene coding for Beta galactomisase and so, to alter the concentration of the enzyme in its cell it must regulate the transcription of the gene

Question 6

What does the lac operon consist of?

Answer 6

• The lac operon consists of a cluster of three structural genes and a length of DNA including operator and promoter regions. The three structural genes are:
  1. lacZ, coding for Beta-galactosidase
  2. lacY, coding for permeate (which allow lactose to enter the cell)
  3. lacA, coding for transacetylase
• Close to the promoter, but not actually part of the operon is its regulatory gene

Question 7

What is the sequence of events when there is no lactose in the medium in which the bacterium is growing?

Answer 7

1. The regulatory gene codes for a protein called a repressor
2. The repressor binds to the operator region, close to the gene for Beta-galactosidase
3. In the presence of bound repressor at the operator, RNA polymerase cannot bind to DNA at the promoter region
4. No transcription of the three structural genes can take place

Question 8

What is special about the repressor protein?

Answer 8

1. The repressor protein is allosteric
2. This means that is has two binding sites
3. When the protein binds to a molecules at one site, this affects its ability to bind to a different molecule at the other binding site
4. The site that binds to DNA is operate from the site that bonds to lactose
5. When lactose binds to its site, the shape of the protein changes so that the DNA-binding site is closed

Question 9

What happens when lactose is present in the medium in which the bacterium is growing?

Answer 9

1. Lactose is taken up by the bacterium
2. Lactose binds to the repressor protein, distorting its shape and preventing it form binding to DNA at the operator site
3. Transcription is no longer invited and messenger RNA is produced from the three structural genes. The genes have been switched on and are transcribed together

Question 10

Why is this mechanism important?

Answer 10

1. The mechanism allows the bacterium to produced Beta-Galactosidase, permeate and transacetylase only when lactose is available in the surrounding medium and to produce them in equal amounts
2. It avoids the waste of energy and materials in producing enzymes for taking up and hydrolysing a sugar tray the bacterium may never meet
3. However the sugar can br hydrolysed when it is available
4. The enzyme Beta-galactosidase is an inducible enzyme

Question 11

Why does the bacterium use glucose rather than anything else?

Answer 11

1. The bacterium uses glucose in preference to other sugars
2. When a bacterium finds both glucose and lactose in the medium in which it is growing, it represses the use of lactose by suppressing the lac operon by means of a different transcription control factor

Question 12

What is gene control in eukaryotes like?

Answer 12

1. In general, the number of different proteins that act as transcription factors increases with increasing size of the genome
2. This means that eukaryotes have many more ways of regulating gene expression than have prokaryotes. In human for example, about 10% of the genes code for proteins which act as transcription factors
3. The factors may bind to the promoter region of a gene, they may increase or decrease the transcription of the gene
4. Whatever the mechanism, their role is to make sure that genes are expressed in the correct cell at the correct time and to the correct extent

Question 13

What are the effects of transcription factors?

Answer 13

1. General transcription factors are necessary for transcription to occur. They form part of the protein complex that binds to the promoter region of the gene concerned
2. Other factors activate appropriate genes in sequence, allowing the correct pattern of development of body regions
3. A transcription factor is responsible for the determination of sex in mammals
4. Transcription macros allow responses to environmental stimuli, such as switching on the correct genes to respond to high environmental temperatures
5. Some transcription factors, including the products of proto-oncogenes and tumour suppressor genes regulate the cell cycle, growth and apoptosis (programmed cell death)
6. Hormones have their effect through transcription factors

Question 14

What is gibberellin?

Answer 14

1. The plant hormone, gibberellin, controls seed germination in plants such as wheat and barley by stimulating the synthesis of amylase
2. It is a good example of how a hormone can influence transcription
3. It has been shown that, in barely seeds, application of gibberellin causes an increase in the transcription of mRNA coding for amylase

Question 15

What is DELLLA and gibberellin?

Answer 15

1. Gibberellin has this effect by causing the breakdown of DELLA proteins
2. A DELLA protein inhibits the binding of a transcription factor, such as phytochrome-interacting protein (PIF), to a gene promoter
3. By causing the breakdown of the DELLA protein, gibberellin allows PIF to bind to its target promoter
4. Transcription fo the gene can then take place, resulting in an increase in amylase production